Preheat oven to 400°F and line a baking sheet with parchment paper.
Slice the baguette in half lengthwise and place cut-side up on the baking sheet. (Optional: lightly brush with olive oil and pre-toast for 3–5 minutes for extra crispiness.)
Spread BBQ sauce evenly over each half of the bread. Top evenly with diced chicken, red onion, and shredded Monterey Jack cheese.